The invention pertains to inductive coils. In particular, it pertains to micro-coils in planar substrates, and more particularly, to three-dimensional micro-coils in such substrates.
Micro-coils on planar substrates in the art are two-dimensional, wherein the operation of them results in eddy current losses in the substrate. Other micro-coils are three-dimensional plated metal structures whose height is limited and are difficult to fabricate uniformly. Three-dimensional micro-coils also are fabricated on small rods and ceramic blocks; however, it is difficult to fabricate large numbers of such devices and integrate them with electronics on planar substrates.
There are micro-coils that consist of spiral inductors fabricated on planar substrates, three-dimensional coils fabricated on the surfaces of tubes, ceramic blocks, or other substrates with cylindrical symmetry, and inductors formed by plating metal structures with high aspect ratios onto substrates.
There are spiral inductors on planar substrates. This is a type of inductor that is fabricated by deposition and photolithographic processes. One example of its use is to increase the magnetic flux coupled into a magnetometer. Its most serious disadvantage is that a substantial fraction of the stored magnetic energy is contained in the substrate. Thus, if the substrate has a finite conductivity, as is usually the case for silicon, eddy current losses can be substantial.
There are three-dimensional coils on cylindrical objects. Helical inductors have been fabricated by patterning metal deposited onto a tube, and inductors with a square cross-section have been fabricated by laser patterning of metal deposited onto an aluminum oxide rod having a square cross-section. The fabrication processes for these devices are not conducive to batch fabrication, and cannot be easily integrated with the fabrication processes for integrated circuits.
Also, there are high-aspect-ratio plated metal inductors. These devices consist of air bridges of thick metal formed on a patterned metal layer on the surface of a wafer. Many air bridges can be connected electrically to form multi-turn air-core inductors whose stored magnetic energy lies mostly outside the substrate. The air bridges are formed by electroplating metal into molds formed from thick photoresist. These inductors can have low eddy current losses, and the fabrication process can be integrated with silicon integrated circuit fabrication. However, the height of the plated structures is limited to the thickness of the photoresist, typically a maximum of 50 to 100 microns, thus limiting the height of the inductor. Also, the thickness of the electroplated metal is non-uniform over the surface of a wafer. This reduces the fabrication yield, and causes the dimensions of the structures, and therefore the electrical characteristics, to vary over the surface of a wafer.
The present invention depicting three-dimensional micro-coils in a substrate avoids the above-noted disadvantages.
The present invention has applications to portable magnetic resonance sensors and analyzers. Applications to other areas of high frequency electronics include low-loss tuned resonant circuits and filters in radio frequency (RF) wireless communication electronics. The preferred fabrication method for the present invention starts by etching a trench in a wafer substrate to define the air core of the inductor. Metal is then deposited onto the trench and patterned, followed by soldering a second wafer to the first wafer to complete the electrical connections for the inductor windings. With this fabrication method, one-turn inductors having a tubular topology may be fabricated. The magnetic field produced by such an inductor is confined mostly to the interior of the inductor. Thus, eddy current losses in the substrate can be minimized, resulting in the fabrication of high Q resonators.
Micro-coils may be components of micro-resonators. The invention covers various types of three-dimensional micro-coils as well as electrical resonators formed from these kinds of micro-coils in planar substrates. The resonators typically operate at VHF, UHF or microwave frequencies.
Micro-resonators having three-dimensional, single-turn tubular micro-coils have been successfully fabricated on silicon wafers. The Q of these resonators is typically about 30 at a resonant frequency of 680 MHz. The inductance of one of these micro-coils is about 0.2 nano-henry (nH). Micro-resonators having micro-coils with two turns and three turns have also been successfully fabricated in silicon wafers. The wafers may be planar substrates made of various materials such as GaAs besides silicon. These multi-turn devices have higher inductance than the one-turn devices, but have a substantially lower Q (Q of about 7 at 432 MHz and Q of about 9 at 545 MHz). The lower Q is caused by the RF magnetic field between the windings penetrating into the silicon substrate, producing eddy current losses in the silicon substrate. A wide range of micro-coil inductances (and hence, resonant frequencies) can be obtained by changing the dimensions of the micro-coils. The advantages of the present invention are noted. The micro-coils are batch fabricated by processes compatible with integrated circuit fabrication techniques. Thus, the micro-coils can be fabricated in large quantities at low cost and integrated with active electronic circuitry. The coils have low eddy current losses because they have an air core. The three-dimensional geometry confines the magnetic field to the inside of the coil, thus minimizing eddy current losses in the substrate or other surrounding conductive materials. The height of the air core, determined by the depth of the etch trench, can be as large as the thickness of the substrate wafer, which is typically 500 microns for a 4-inch silicon wafer, and much thicker for the larger wafer diameters typically used in integrated circuit fabrication.
The inductance depends on the dimensions of the etched trench, and the shape of the patterned metal in the etch trench. The dimensions of the etched trench can be uniform for many devices over the surface of a wafer, and the shape of the patterned metal is determined by well-defined photolithographic processes.
Three-dimensional micro-coils have applications in miniature magnetic resonance spectrometers used as sensors and analyzers. Nuclear magnetic resonance (NMR), electron spin resonance (ESR), or nuclear quadropole resonance (NQR) can be measured with such a device. Magnetic resonance spectroscopy is a powerful tool for detection and identification of chemical species. An electron spin resonance (ESR) signal is typically caused by a free radical, and hence is sensitive to the chemical environment. An NMR signal is typically affected by small frequency shifts due to neighboring nuclei and electrons. Thus, each nucleus in a molecule will have a slightly different magnetic resonance frequency. As a result, a complex molecule can have a unique NMR spectrum.
The greatest obstacle to miniaturization of magnetic resonance spectrometers is the size of the magnet providing the DC field needed to polarize the specimen being measured. A large, uniform polarizing magnetic field is desirable in order to achieve high signal to noise and narrow magnetic resonance linewidth. A typical laboratory ESR spectrometer uses a magnet weighing over 1000 kilograms, which provides a uniform field of approximately 0.3 Tesla over a pole-piece diameter of several inches. A typical laboratory NMR spectrometer uses a superconducting magnet providing a field of order 10 Tesla or more. If the size of the pick-up coil can be reduced, then the diameter of the magnet""s pole pieces and the gap between the pole pieces can be reduced, thus allowing the volume of the entire magnet to be dramatically reduced. The gap between the pole pieces is important because the number of amp-turns required to achieve a given magnetic field is approximately proportional to the gap spacing. Thus, a small gap reduces the size of the magnet windings and the power supply requirements. The present invention permits the micro-coil thickness, and hence the gap between the pole pieces, to be about one millimeter. The diameter of the pole pieces would be about two centimeters, which is a few times larger than the typical length of the micro-coil. Such a magnet is small enough to allow construction of a handheld magnetic resonance analyzer.
There are further advantages of the present invention for use in miniature magnetic resonance spectrometers. The signal to noise ratio per magnetic resonant spin is higher for small pickup coils than for large pickup coils. Thus, for analyzing very small samples, small coils provide the optimum signal to noise. Also, micro-coils on planar substrates permit inexpensive integration of the pickup coil with the signal processing electronics.
Analyzers with multiple pickup coils are more cost effective with all the coils integrated onto a single substrate, as made possible by the present invention. Integration of the pickup coils with micro-fluidic gas and liquid sampling systems and other microanalysis systems is facilitated.
The invention has applications for miniaturized wireless communications circuitry. On-chip integrated inductors allow more design flexibility and easier fabrication of filters and tuned resonant circuits at UHF, VHF and microwave frequencies. Such inductors also have applications in microprocessors, especially as clock speeds increase toward one GHz and beyond.
This invention makes possible the fabrication of arrays of resonant circuits. The resonant circuits can be fabricated by batch fabrication processes. Many of these circuits can be fabricated on a single planar substrate simultaneously. Photolithographic patterning allows the dimensions of each resonant circuit to be precisely defined, therefore providing accurate control of each resonant frequency as well as the properties of circuits that couple energy between them. One application of such an array of resonant circuits would be to form a resonator with flat frequency response over a specified frequency range. Several resonant circuits, each with a slightly different resonant frequency, would be electrically coupled to each other to provide the desired flat frequency response. The coupling would be performed by transmission lines consisting of patterned dielectric and metal layers on one or both of the planar substrates. A transmission line could be connected directly to the capacitor of each resonant circuit, or to a secondary inductor formed near the primary inductor of each resonant circuit so that the mutual inductance between the secondary and primary inductors provides coupling of energy between the transmission line and the resonant circuit.
A resonator formed from an array of several coupled resonant circuits can be used as an electrical filter having a flat band-pass response. The flat frequency response would also be advantageous for use as the pick-up coil in an NMR or ESR spectrometer. Precise dimensional control is essential for fabrication of such a device, in order to control the resonant frequencies of the individual resonant circuits and the characteristics of the coupling circuitry connecting them together. Batch fabrication using photolithography allows such devices to be built at relatively low cost. Other batch fabrication processes on planar substrates, such as screen-printing, can be used when the device dimensions are large enough to allow such processes. The invention may be fabricated on flexible or rigid planar substrates. Flexible substrates can include polyimide, such as KAPTON, or other polymers.